1. Field of the Invention
The present invention relates to a circuit for converting an electric voltage into an electric current, and more specifically to a current output circuit in a voltage-current converting circuit which uses an operational amplifier.
2. Description of Related Art
One typical example of conventional voltage-current converting circuits using an operational amplifier has been combined with a current switch. The voltage-current converting circuit includes an operational amplifier having a non-inverting input connected to receive an input voltage, a voltage-current converting transistor having a base connected to an output of the operational amplifier, and a pair of current switching transistors connected in series to the converting transistor, so that a current is outputted from a selected one of the pair of current switching transistors. With this arrangement, regardless of which of the pair of current switching transistors is selected, the output current flows in series through the voltage-current converting transistor and the selected one of the pair of current switching transistors. As result, the output current of the conventional voltage-current converting circuit is greatly influenced by a current amplification factor of the voltage-current converting transistor and a current amplification factor of the current switching transistors in the form of multiplication. Therefore, the conventional voltage-current converting circuit has a large error caused by variation of the current amplification factors. In order to depress the error caused by the current amplification factors, if each of the voltage-current converting transistor and the current switching transistors is put in the form of a Darlington connection, three transistors have to be added.